1. Field of the Invention
The present invention relates to a laser scanning unit for an image forming and/or reproducing apparatus, and more particularly, to a laser scanning unit in which a motor drive chip for a polygonal mirror motor is installed outside of a housing.
2. Description of the Related Art
Laser scanning units are employed in a printing machines such as, for example, laser printers. A conventional laser scanning unit includes: a laser source; a movable mirror; and a lens system. Generally, the laser source emits laser beams which are directed by the mirror and the lens system to a surface of a charged photoconductive medium, such as a photoconductive drum or a photoconductive belt. The lens system compensates for any image distortion caused by, for example, the varying distance between the mirror and points along the photosensitive drum or belt. The laser beam changes the charge of portions of the photoconductive medium on which it is incident forming a latent image on the photoconductive medium which corresponds to the image to be printed and to which toner may adhere.
FIG. 1 is an exploded view illustrating the internal configuration of a conventional laser scanning unit.
Referring to FIG. 1, the conventional laser scanning unit includes various optical elements. The optical elements include a laser diode (LD) 11 emitting a laser beam, a collimating lens 12 collimating a laser beam emitted from the LD 11 so that the laser beam is parallel to or lined up with an optical axis, a polygonal mirror 14 horizontally moving a laser beam which has passed through the collimating lens 12 at a constant linear speed, a cylindrical lens 13 imaging a laser beam on a surface of the polygonal mirror 14 in a horizontally linear shape, Fθ lenses 15 having a refractive index with respect to the optical axis which lenses polarize a laser beam reflected by the polygonal mirror 14 at a constant speed to a main scanning direction and correcting aberration to focus the laser beam on a scanned surface, an image-forming mirror 16 reflecting a laser beam which has passed through the Fθ lenses 15 and imaging the laser beam in the form of dots on a surface of a photoconductive drum 60 of a printing machine, an optical sensor 18 receiving a laser beam and providing a horizontal synchronization, and a synchronization signal detecting mirror 17 reflecting a laser beam to the synchronization signal detecting optical sensor 18. Such optical elements are, as illustrated, often installed inside a housing 50 and sealed so as not to be contaminated by foreign substances, such as dust or toner.
A motor 20 rotating the polygonal mirror 14 at a constant speed is installed on a circuit board 30 within the housing 50. A motor drive chip 40 formed of a semiconductor integrated circuit is mounted on the circuit board 30 to drive and control the motor 20. A circuit board 10 controlling the LD 11 is disposed inside the housing 50.
FIG. 2 is a block diagram illustrating the circuit configuration of the motor drive chip of the conventional laser scanning unit of FIG. 1.
Referring to FIG. 2, the motor 20 rotating the polygonal mirror 14 at a constant speed includes three position sensors 21, 22, and 23, and a speed sensor 24. In general, hall sensors are used as the sensors 21, 22, 23, and 24. The motor drive chip 40 includes a position signal amplifying section 41, a speed signal amplifying and filtering section 42, a speed control section 43, a commutation control section 44, and a three-phase inverter 45. The sensors 21, 22, and 23 are each connected to the position signal amplifying section 41 of the motor drive chip 40 by two signal lines. The speed sensor 24 is connected to the speed signal amplifying and filtering section 42 by two signal lines. The three-phase inverter 45 is respectively connected to terminals u, v, and w of the motor 20 (shown in FIG. 1) by three power supply lines.
The position signal amplifying section 41 amplifies position signals Sa, Sb, and Sc of a rotor of the motor 20 (shown in FIG. 1) respectively received from the position sensors 21, 22 and 23 and transmits the amplified signals to the commutation control section 44. The speed signal amplifying and filtering section 42 amplifies and filters a speed signal Sd received from the speed sensor 24 and transmits the amplified and filtered signal to the speed control section 43. The speed control section 43 calculates a control signal to control the rotation speed of the motor 20 in response to the received speed signal and transmits the control signal to the commutation control section 44. The commutation control section 44 controls the three-phase inverter 45 in response to the received position signal and the speed control signal. The inverter 45 respectively supplies current in a proper switching order to the terminals u, v, and w of the motor 20 so that the motor 20 rotates at a constant speed.
In the conventional laser scanning unit described above, the motor drive chip 40 is disposed inside the housing 50. Further, the motor drive chip 40 acts as a heat-source during operation. As a result, during operation, the temperature inside the laser scanning unit increases due to heat generated by the motor drive chip 40. Properties of the LD 11 and the Fθ lens 15 are temperature sensitive. Consequently, the temperature increase inside the laser scanning unit affects properties of the LD 11 and the Fθ lens 15.
Tables 1 and 2 present measurement results of internal temperature changes and temperature changes in each element in the conventional laser scanning unit. Table 1 shows temperature changes (in ° C.) in each element of the laser scanning unit over time when a motor is continuously driven at 22,000 rpm under low temperature/humidity conditions. Table 2 shows temperature changes (in ° C.) in each element in the laser scanning unit over time when a motor is continuously driven at 22,000 rpm under high temperature/humidity conditions.
TABLE 1SurfaceExternalSurfaceBottomSurfacetemperatureSurfaceTimetemperatureLSU Internaltemperaturetemperature oftemperatureof collimatingtemperature(Min.)(° C.)temperatureof drive chipmotorof Fθ lenslensof LD case023.933.642.440.028.530.530.51024.034.143.640.328.730.631.02024.344.957.150.237.443.545.63024.050.455.656.142.449.451.44025.354.358.160.245.853.255.15023.957.662.062.848.656.558.56023.960.264.165.251.859.361.37024.061.465.266.353.060.361.68023.961.264.766.552.860.262.09024.560.964.966.152.860.162.010024.460.864.366.153.260.061.611024.061.264.866.253.360.362.312024.161.765.366.853.960.862.713024.262.866.567.554.761.863.714024.262.666.167.755.461.763.3
TABLE 2SurfaceExternalSurfaceSurfaceSurfacetemperatureSurfaceTimetemperatureLSU Internaltemperaturetemperature oftemperatureof collimatingtemperature(Min.)(° C.)temperatureof drive chipmotorof Fθ lenslensof LD case 032.150.349.048.250.249.150.01032.250.160.648.149.448.449.42032.659.773.861.553.757.659.93033.664.578.967.958.562.965.14033.267.877.470.661.666.267.65033.668.983.472.463.867.869.96033.269.884.573.365.168.971.07034.570.384.873.565.769.471.48033.570.385.073.865.969.571.69033.370.679.872.665.569.069.7
It can be seen from Tables 1 and 2 that the internal temperature increase in the laser scanning unit is slightly affected by the environment in which the laser scanning unit is used as well as conditions of use of the laser scanning unit. However, as the external temperature increases, the temperature of each element of the laser scanning unit also increases. Further, as a drive time becomes longer, the degree of temperature increase in each element increases. In particular, the surface temperature of the motor drive chip exhibits the largest increase, and the surface temperature of the polygonal mirror motor exhibits the second largest increase. Therefore, it is shown that the greatest cause of the internal temperature increase of the laser scanning unit is the motor drive chip.
The internal temperature increase of the laser scanning unit due to heat generated by the motor drive chip leads to the temperature increase of the laser diode. Consequently, the temperature characteristic of the laser diode changes and, as a result, the optical power of the laser diode cannot be controlled with precision.
Further, the internal temperature increase of the laser scanning unit causes the temperature of the Fθ lens to increase. The temperature increase of the Fθ lens, which is typically injection molded plastic, affects the refractive index and curvature of each region in the Fθ lens. As a result, variation of the diameter of the optical spot formed on a surface of the photoconductive medium increases.
Table 3 presents measurement results of diameters of an optical spot with changes in the temperature inside the laser scanning unit. Positions of the optical spot, that is, 0,−100, and 100 mm, represent the center of a scanning line and distances from the center to both ends of the scanning line, respectively, and −2 mm˜+2 mm represents changes in the length of the Fθ lens with temperature changes. “Main” and “sub” represent diameters of a main scanning direction and a sub scanning direction of the optical spot, respectively.
TABLE 3LSUPosition−2 mm−1 mm0 mm + 1 mm+2 mmInternalof opticalmainsubmainsubmainsubmainsubmainsubOpticaltemperaturespot(μm)(μm)(μm)(μm)(μm)(μm)(μm)(μm)(μm)(μm)power24.5° C.−1007379727974798783115860.1920697670777177748180840.3901007071727378778783123990.21935.0° C.−10072817781818110283126860.2250707771777478798190830.2451007373727580789684134940.19045.0° C.−10077848383968212586140900.21907075727776778680102830.241100747181741057612684143930.19255.0° C.−100747980791007913081144830.223073767776897611878132810.239100777186721157514079147830.18165.0° C.−100787693741147614784186890.1920798189781147813178140810.239100657792821267914485157930.214
It can be seen from Table 3 that as the temperature of the Fθ lens increases, the diameters, in scanning direction and the sub scanning direction, of the optical spots formed on the surface of the photoconductive medium increase by 30˜40 μm or more. As the diameter of th optical spot formed on the surface of the photoconductive medium and the diameter variation increase, the resolution and uniformity of an image decrease.
One approach to addressing the above-described disadvantages is to dispose the motor drive chip 40 outside of the housing 50 so that the motor drive chip 40 is isolated from the LD 11 and the Fθ lenses 15. However, locating the motor drive chip 40 outside of the housing exposes the signal and power supply lines to outside interference. As shown in FIG. 2, a plurality of signal lines which respectively connect the motor drive chip 40 to the sensors 21, 22, 23, and 24 included in the motor 20 and the power supply lines which supply electric power to the motor 20 are exposed outside the housing 50. Thus, disadvantageously, severe noise is generated due to an] by electromagnetic fields outside the housing 50. Further, the position and speed sensors are typically low voltage sensors such as, for example, hall sensors which output sine wave signals of approximately ±0.1˜0.2V. Since the sensor output voltage is low, the output signals are very sensitive to noise, and accordingly, in the conventional art, the motor drive chip 40 needs to be positioned as close as possible to the position and speed sensors 21, 22, 23, and 24.